Field of the Invention
The invention relates to an optical reader including a multi-channel detector.
Various readers and optical scanning systems have been developed for reading printed indicia such as bar code symbols appearing on a label or the surface of an article and providing information concerning the article such as the price or nature of the article. The bar code symbol itself is a coded pattern of indicia comprised of, for example, a series of bars of various widths spaced apart from one another to form spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers electro-optically transform the graphic indicia into electrical signals which are decoded into alpha-numeric characters that are intended to be descriptive of the article or a characteristic thereof. Such characters typically are represented in digital form, and utilised as an input to a data processing system for applications in point of sale processing, inventory control and the like.
Known scanning systems comprise a light source for generating a light beam incident on a bar code symbol and a light receiver for receiving the reflected light and decoding the information contained in the bar code symbol accordingly. The readers may comprise a flying spot scanning system wherein the light beam is scanned rapidly across a bar code symbol to be read or a fixed field of view reading system wherein the bar code symbol to be read is illuminated as a whole and a CCD (Charge Coupled Device) array is, provided for detecting the light reflected from the bar code symbol. The reader may be either a hand-held device or a surface-mounted fixed terminal.
A variety of scanning devices are known. The scanner could be a wand type reader including an emitter and a detector fixedly mounted in the wand, in which case the user manually moves the wand across the symbol. Alternatively, an optical scanner scans a light beam such as a laser beam across the symbol, and a detector senses the light reflected from the symbol. Alternatively a gun-type hand-held arrangement may be provided. In either case, the detector senses reflected light from a spot scanned across the symbol, and the detector provides the analog scan signal representing the encoded information.
A digitizer processes the analog signal to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The threshold level effectively defines what portions of a signal the reader will recognize as a bar or a space.
Readers of the type discussed above are single channel systems having a single digitizer output and/or a single processing chain to produce a single digitized output.
The pulse signal from the digitizer is applied to a decoder. The decoder first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyses the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard the scanned symbol conforms to. The recognition of the standard is typically referred to as auto discrimination.
Different bar codes have different information densities and contain a different number of elements in a given area representing different amounts of encoded data. The denser the code, the smaller the elements and spacings. Printing of the denser symbols on a appropriate medium is exacting and thus is more expensive is than printing low resolution symbols.
A bar code reader typically will have a specified resolution, often expressed by the size of its effective sensing spot. The resolution of the reader is established by parameters of the emitter or the detector, by lenses or apertures associated with either the emitter or the detector, by the threshold level of the digitizer, by programming in the decoder, or by a combination of two or more of these elements.
In a laser beam scanner the effective sensing spot may correspond to the size of the beam at the point it impinges on the bar code. In a scanner using an LED or the like, the spot size can be the illuminated area, or the spot size can be that portion of the illuminated area from which the detector effectively senses light reflections. By what ever means the spot size is set for a particular reader, the photodetector will effectively average the light detected over the area of the sensing spot.
A high resolution reader has a small spot size and can decode high density symbols. The high resolution reader, however, may have trouble accurately reading low density symbols because of the lower quality printing used for such symbols. This is particularly true of dot matrix type printed symbols. The high resolution reader may actually sense dot widths within a bar as individual bar elements. In contrast, a low resolution reader has a large spot size and can decode low density symbols. However, a reader for relatively noisy symbols such as dot matrix symbols reads such a wide spot that two or more fine bars of a high resolution symbol may be within the spot at the same time. Consequently, a reader having a low resolution, compatible with dot matrix symbols can not accurately read high density symbols. Thus any reader having a fixed resolution will be capable of reading bar codes only within a limited range of symbol densities.
For a given symbol density, the resolution of the reader also limits the range of the working angle, i.e. the angle between the axis of the reader and a line normal to of the surface on which the bar code is printed. If the range and resolution are too limited, a user may have difficulty holding a hand-held reader comfortably while accurately scanning the bar code. This can be particularly troublesome if the reader incorporates additional elements to form an integrated data terminal. The combination of size, weight and an uncomfortable angle can make reading in large amounts of bar code information difficult and annoying, and thereby make the user more resistant to use of the bar code system. Problems also arise with fixed readers which may have to be manually switched dependent on the resolution required, reducing efficiency and slowing operation of the system.
One solution might be to provide some means to adjust the resolution or sensing spot size of the reader, e.g., by adjusting the threshold of the digitizer. This approach, however, would require a number of different scans at different resolutions. If the scan is automatic, the variation in resolution causes a loss of robustness because the scan is at the correct resolution only a reduced amount of the time. Effectively such a scanner would scan at the equivalent of a reduced rate. If the reader is a hand-held device, the user would have to manually scan the reader across the information each time the resolution changes. This causes a marked reduction in the first read rate and increased frustration for the user.
In addition most optical scanners such as bar code scanners are adapted for use at a particular distance, or a range of distances, from an indicia to be scanned. If the user holds the scanner too close to the indicia, (or, conversely the object is held too close to the scanner) or too far away, the indicia and/or the flying spot beam will not be in focus, and decoded will not be possible.
Such scanners may not be particularly convenient in environments where a series of indicia to be read are presented to the scanner at various distances, and where it is difficult or impossible for the user to alter the distance between the scanner and the indicia. To deal with such situations, attempts have been made to expand the acceptable working range of conventional scanners, to give the user as much leeway as possible, and also to provide multi-distance scanners which can operate, for example, at a first working range or at a second working range according to the user""s preference or requirements. One possibility is for the provision of a two-position switch on the scanner, with the scanner operating at a first working distance in a first position of the switch and at a second working distance in a second position.
An improved solution is to provide an optical scanner including a multi-surface reflector having a first surface of a first profile and a second surface of a second profile, the scanner being adapted to read an indicia at a first distance or working range when the beam is reflected from the first surface and at a second distance or working range when the beam is reflected from the second surface.
A further problem associated with conventional methods of signal processing is that the analog signal is processed only once and the decodeability of the processed signal is principally determined by the signal to noise ratio of the incoming signal.
In addition known digitizers have been recognised as requiring improvement in the scanning of dot matrix symbols such as bar code symbols printed in dot matrix format. The failure to decode such symbols is commonly caused by a split in a wide bar wherein a void in the bar causes a narrow element to appear in the middle of the bar. This only happens in the case of wide bars because such bars are built up of two or three columns of dots and small spaces between the columns are sometimes large enough to be recognised and digitized as wide elements. It will be appreciated that the problem does not arise with narrow bars made up a single column of dots.
It is an object of the present invention at least to alleviate the problems of the prior art. It is a further object to provide a reader allowing improved reading of printed indicia at multiple resolutions.
It is a further object still of the present invention to provide improved reading of printed indicia presented at various distances from the reader.
It is still a further object of the invention to provide an optical reader having an improved signal to noise ratio.
It is yet a further object of the invention to provide further improved reading of dot matrix symbols such as bar code symbols.
According to the invention there is provided an optical reader for generating an outgoing light beam to illuminate an information symbol comprising regions of different light reflectivity, and for collecting an incoming light beam reflected from the symbol, the reader comprising:
a source of the outgoing light beam;
light collection optics for collecting the incoming light beam and for directing the incoming light beam to a multi-channel photodetector;
the multi-channel photodetector comprising an array of more than one individual active optical sensing element, each said element having an output capable of providing an output signal representative of light impinging thereon;
the output of each detection element being coupled to a respective channel for processing the output signal of a respective detection element. Because a plurality of channels are involved, improved coupling out of signals at each channel is achieved. This is of particular advantage when it is desired to convey different signal information in each channel.
One of the active optical sensing elements may be positioned around another of the active optical sensing elements. Preferably the outgoing light beam, light collection optics, and array of sensing elements are configured so that the output of a first group of the sensing elements corresponds to a spot of a first size passing across the information symbol, and so that the output of a second group of the sensing elements corresponds to a spot of second size, larger than said first size, passing across the information symbol. Accordingly each channel will transfer a signal at a different resolution level.
The array of sensing elements may comprise a first and a second sensing element, and the first sensing element may be contained within the first group and the first and the second elements are contained within the second group. The first group may contain only the first sensing element, and the second group may contain only the first and second sensing elements.
According to the invention there is further provided a reader for reading printed indicia, for example bar code symbols, comprising a light source for illuminating an indicia and a light detector for producing a signal corresponding to detected light reflected from the indicia, the light detector having a plurality of channels, each channel being associated with a different resolution level of the signal produced by the detector. Thus symbols or indicia at different resolutions can be read quickly and simply by a single reader.
The detector may be arranged to process the signal and transfer a signal processed at a first threshold level via a first channel and a signal processed at a second resolution level via a second channel.
Alternatively the light source may be arranged to generate a first illuminating beam at a first resolution and a second illuminating beam at a second resolution, the first and second illuminating beams being spatially separated, and the detector may comprise a first region associated with a first channel for producing a signal corresponding to the first beam and a second region associated with a second channel for producing a signal corresponding to the second beam. The light source may be arranged to generate a first and second illuminating beam spaced laterally from one another and the detecting regions of the detector corresponding to the first and second beams may be laterally spaced from one another. Alternatively the light source is arranged to generate concentric first and second illuminating beams and the detecting regions of the detector corresponding to the first and second beams may be concentric.
The light source may be arranged to generate a light beam of a first resolution at a first wavelength and a light beam of a second resolution at a second wavelength, and the detector may be arranged to detect light of the first wavelength and produce a signal corresponding to the first light beam at a first channel and to detect light of the second wavelength and produce a signal corresponding to the light of the second wavelength at a second channel.
The light source may be arranged to generate a light beam at a first resolution in a first set of pulses and a light beam at a second resolution in a second set of pulses temporally off-set from the first set of pulses, and the detector may be synchronised with the light source to detect light pulses of the first resolution and produce corresponding signals at a first channel and to detect the light pulses of the second resolution and produce corresponding signals at a second channel.
The reader may comprise a flying spot scanning reader or a field of view optical reader. A decoder may be arranged to analyse the signal from a channel to determine whether it is a valid signal and arranged to merge the signals from each channel if no valid signal from an individual channel is detected.
According to the invention there is further provided a reader for reading printed indicia comprising a light source for illuminating an indicia and a light detector for detecting light reflected from the indicia wherein the detector comprises a first region and a second region for detecting reflected light at a first resolution level and a second resolution level respectively, a first channel being associated with the first region and a second channel being associated with the second region.
According to the invention there is further provided a method of operation of a reader for reading printed indicia comprising a light source for illuminating the indicia and a light detector producing a signal corresponding to detected light reflected from the indicia, a first channel and a second channel being associated with the detector for carrying respective signals at different resolution levels, and a decoder; the method including the steps of decoding the signal from the first channel and, if that signal is not valid, decoding the signal from the second channel and, if that is not valid, merging the signals from the first and second channels, decoding the merged signal and selecting the valid portions of the merged signal.
According to the invention there is further provided a reader for reading printed indicia such as bar code symbols comprising a light source for illuminating an indicia and a detector for producing a signal corresponding to detected light reflected from the indicia, a first channel and a second channel being arranged to receive signals processed by the detector wherein the second channel includes buffer means for delaying a signal and merging the signal with a subsequent signal transferred by the first channel. As a result, the signal portion is enhanced and the noise portion reduced giving rise to an improved signal to noise ratio.
An indicia may be scanned at intervals T giving rise to a signal to the first and second channels via the detector, and the buffer means of the second channel may be arranged to delay the signal for the period T in order to merge the signal with the subsequent signal transferred by the first channel.
According to the invention there is further provided a method of processing analog signals corresponding to a light beam reflected from a printed indicia including receiving the light beam at a detector, converting the light beam to a digital signal and transferring the digital signal via a first channel to a decoder, transferring the digital signal from the detector via a second channel to a delay buffer and merging the delayed signal with a subsequent signal transferred by the first channel.
According to the invention there is further provided a method of decoding a bar code symbol composed of elements of different reflectivity comprising scanning the bar code symbol with a light beam, detecting the reflected light beam and providing a digitized signal carrying the bar code information, wherein the decoder compares the width of each element of a first reflectivity with the width of another element of that reflectivity in the bar code symbol and identifies the element as a defect if its width is less than a predetermined proportion of the width of the other element. As result, defects in a printed bar code symbol, for example because of defectively printed dark regions, may be detected.
The predetermined proportion may be 60%.
If the element is identified as a defect its width may be added to the width of the elements on either side of the element. It is thus possible to determine the width, for example, of a bar, if the intermediate element is a space.
The element may be compared with neighbouring elements of the same reflectivity.
According to the invention there is further provided a processor for processing an optical signal received from a printed indicia, the processor including a detector, a first channel and a second channel associated with a detector and a decoder wherein the channels are arranged to transfer signals at different resolution levels from the detector to the decoder.